Low cycle fatigue behavior and mechanisms of a eutectic Sn–Pb solder 63Sn/37Pb

Low cycle fatigue tests of as-casted Sn–Pb eutectic solder (63Sn/37Pb) were carried out using the non-contact strain controlled system at 20°C. The fatigue behavior followed the Coffin–Manson equation with the fatigue ductility exponent of 0.63. Without local deformation and stress concentration at contact points between the extensometer and the specimen surface in strain-controlled fatigue tests, the crack initiation and propagation were studied by SEM examination of specimen surface with replication technique, longitudinal cross section and fracture surface. Colony boundary sliding and microcracks along colony boundaries were detected in the early stage of fatigue life. Cavities on colony boundaries, especially on the interphases between Sn–Pb phases, and wedge cracks at the triple-point colony coners were the initiation sites of microcracks. Stage II crack propagated in intergranular manner along the eutectic colony boundaries. This propagation of stage II cracks could be expressed by the relation of dac/dN=1.6×10−10 [ΔJ]1.2, where ac is average crack length and ΔJ is J-integral range. After fatigue tests, small grains were observed in Sn-rich phases near fracture surface.